How Finless Porpoises Hear In The Murky, Noisy Yangtze
The Yangtze is the longest river in Asia and it’s also one of the busiest. The amount of traffic in the Chinese waterway can pose a significant problem for animals living there, and a new study by scientists from the Woods Hole Oceanographic Institution (WHOI) in Massachusetts has found that the Yangtze’s finless porpoises may have trouble using sound to negotiate their way through the bustling waters.
With an estimated 1,000 animals alive today, many are worried the finless porpoise could follow the fate of the baiji dolphin – another Yangtze mammal that was declared effectively extinct in 2006.
“We want to understand how they may be impacted by noise,” study author Aran Mooney, a WHOI biologist, said about the porpoise.
Marine and freshwater mammals alike rely heavily on their hearing to communicate and find their way in dark and murky waters. According to WHOI researchers, previous studies have been limited to just a few species, bottlenose dolphins in particular, because of their availability in marine parks and aquariums. The Massachusetts marine biologists said this reliance can be an issue because policymakers and regulators could base noise pollution policy decisions on data taken from just a few species rather than a broad range that would account for more sensitive animals.
In the new study, which was published in the Journal of Experimental Biology, WHOI scientists were able to show how inconsistency in the size and shape of toothed cetaceans’ heads across species can affect how they perceive sound and how sensitive they are to a various frequencies.
“We’ve learned that there’s more variation than we’ve taken into account on how different species hear,” Mooney said.
To reach their conclusion, WHOI scientists, in conjunction with a team of Chinese scientists from the Chinese Academy of Sciences, applied acoustic sensitivity examinations and computed tomography (CT) scans on Yangtze finless porpoises to identify the auditory differences between toothed cetaceans.
Like all toothed marine mammals, the Yangtze finless porpoise does not have external ears. It detects sound by picking up reverberations in the the water with its head, throat, jaw and acoustic fat located in its mandible.
Study researchers conducted acoustic examinations on two Yangtze finless porpoises that were originally wild animals, but have been living in captivity at the Institute of Hydrobiology in Wuhan, China for six and 14 years each. The exams were similar to the hearing tests regularly given to human infants.
“Porpoises, like babies, can’t tell us if they can hear in their left or right ear, so we measure their hearing physiologically from the surface of the skin,” Mooney noted.
While audio clicks and various tones were transmitted through silicon suction cup sensors on nine parts of the animal’s head and body, the marine biologists were able to non-invasively scan the porpoises’ brain responses.
“We had the opportunity to scan them in Wuhan and work with the Chinese radiologists, which was very interesting to get a chance to see their facility and how they operate in comparison to WHOI,” said study author Darlene Ketten, director of the Computerized Scanning and Imaging (CSI) facility at WHOI. “We’ve done a lot of these here. But, they had never scanned any porpoises.”
The researchers found the porpoises’ acoustic fat pads are thicker and more disc-like than those found in other toothed whales.
“Now that we have some hearing data, we are working on modeling how the conformation of these pads and their dimensions and shapes relate to the frequencies and sensitivities,” Ketten said.
“In a noisy environment, they’d have a hard time hearing their prey or their friend,” Mooney said. “It makes it more difficult for them to conduct basic biological activities such as foraging, communicating, and navigating in the river.”
He added that effective management strategies must consider the variations in hearing for different marine mammals.